What is the purpose of a hold-down on a back-feed breaker in a main lug panel?
I am installing a sub-panel in an outbuilding. I have a main lug panel and would like to use a two pole breaker as a main disconnect. I am having trouble finding the listed hold-down and was wondering if it is really necessary. What good does it do?
Thanks in advance.

Those hold downs are usually special order BUT why not just get a panel with a main breaker. I know it cost more. You could sell the mlo panel on eBay however.

The hold down is required per NEC because the breaker can easily slip out of the retainer slot because it's being connected backwards style with what are usually bigger fatter wires then the standard hookup. But you see, you are trying to be economical by using what you already have. The NEC has required hold down bolts to mitigate the possibility of breaker "pop out". Why go there in the first place! Just get a panel with the correct size main breaker and you won't need to backfeed on an mlo panel. I think its safer because its standard.

Yea the backfed approach does work but should not be done in my opinion. It reminds me of the piggyback breakers that work but risk overloading panels. Just cause they will sell it to you doesn't mean you should buy it.

Yes, the retainer clip is required, simply because it is the Main Breaker, for reliability and the amount of current the Main is intended to carry, to prevent overheating possible loose power connections.

Not because of "fat" wires.

These retainer kits are popular, backfeeding panels is done often, and usually very available. Not sure what brand your looking for.

Locate a local electrical distributor that handles your brand panelboard.

Many times, for example, a 100 amp panel is used as a subpanel that only needs a 40, 50, or 60 amp Main, so I see no need to order a panel with a Main Breaker, as suggested.